serial: ns16550: Fix return-type warning

serial: ns16550: Fix return-type warning

Fix compiler warning:

drivers/serial/ns16550.c: In function ‘serial_in_dynamic’:
drivers/serial/ns16550.c:153:1: warning: control reaches end
    of non-void function [-Wreturn-type]
  153 | }
      | ^

Observed with gcc 15.2.1:

$ riscv64-unknown-linux-gnu-gcc --version
riscv64-unknown-linux-gnu-gcc (Gentoo 15.2.1_p20260214 p5) 15.2.1

Fixes: 62cbde4c4e46 ("serial: ns16550: Support run-time configuration")
Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Reviewed-by: Tom Rini <trini@konsulko.com>